Spread spectrum modulation refers to any modulation scheme that produces a spectrum for the transmitted signal much wider than the bandwidth of the information being transmitted independently of the information-bearing signal. Spread spectrum modulation has recently been used in digital cellular communication systems, such as the digital cellular communications systems that comply with Telecommunications Industry Association (TIA) Interim Standard (IS) 95.
The IS-95 system has been designed to replace a number of existing analog cellular radio channels with a single direct sequence CDMA carrier. Asymmetric modulation methods have been employed in the forward link (base-to-mobile) and the reverse link (mobile-to-base), although both links are spread at a 1.2288-Mchips/s rate, thereby permitting both links to occupy the same bandwidth. The forward link employs coherent modulation/demodulation. The reverse link employs non-coherent modulation/demodulation.
In the mobile transmitter of the reverse link, the speech coder output is supplied to a rate-1/3 constraint-length-9 convolutional encoder. The output of the convolutional encoder is then applied to a 32.times.18 block interleaver of length 576 symbols. The coded, interleaved symbols are then block coded using a 64-ary orthogonal modulator.
At the output of the 64-ary modulation process, the symbol rate is 28.8 ksym/s.times.64/6=307.2 k-Walsh Chips/s. The coded symbol stream is then spread and scrambled by mixing a masked 1.2288-Mchips/s long code (period 2.sup.42 -1) from a long-code generator. This masked long code may also be referred to as a user code or subscriber code because it differentiates the signal of one from another. This data stream is spread further by simultaneously spreading the data stream in quadature with two short-length (period 2.sup.15) sequences. The resulting quadature channels are then applied to an offset-QPSK modulator for I/Q filtering and for mixing up to the output carrier frequency.
At the base station, the received signal is initially despread with the quadature length 2.sup.15 PN sequences. The resulting signal is then further despread/descrambled with the long user code before being applied to a 64-ary correlator. In the prior art, traditional IS-95 Rake demodulators despread signal data serially, as each signal sample is received. Therefore, prior art Rake demodulators require at least one complete demodulator circuit for each user, and preferably additional demodulators for demodulating fingers, or multipath signals, from each user. A finger may be referred to as a copy of the CDMA signal, which is usually received later in time because of the propagation delay traveling over a longer path.
In addition to duplicating demodulator circuitry, the throughput of the serial despreading architecture is limited by the incoming data rate, which is typically fixed and cannot be increased in order to increase throughput. Therefore, it would be desirable to have a new demodulation architecture that allowed the despread circuitry to operate at a rate faster than the incoming data rate, which increases data throughput.
As shown in FIG. 1 and mentioned above, this serial CDMA demodulation architecture requires many copies of the demodulator and decoder circuits in order to demodulate signals from multiple system users. As depicted, digital wireless communications system 20 receives CDMA signals from subscribers 22 and 24. Subscriber 22 transmits signal A and subscriber 24 transmits signal B, both of which are received by antenna 26. Subscriber 22 is depicted closer to antenna 26 than subscriber 24. This means that a frame, or group of symbols, in signal A arrives at antenna 26 sooner than a frame transmitted from subscriber 24. Moreover, signal A and signal B may overlap in time.
From antenna 26, the radio frequency signals are coupled to receiver 28, which removes the radio frequency carrier from signals A and B.
The output of receiver 28 is then converted from an analog signal to a digital signal by A/D converter 30. The digital data stream output from A/D converter 30 includes the information contained in both signal A and signal B, which are separated from one another by different user codes.
In a typical IS-95 system, A/D converter 30 uses a 4-bit output to represent a range of values in offset 2's compliment form. Also, typical IS-95 systems use dual A/D converters 30--one for an I channel and one for a Q channel. Demodulators 32 use selected user codes to separate the CDMA signals from each other, and decoders 38 decode the encoded symbols to recover subscriber data.
In the prior art, demodulators 32 multiply the output samples of A/D converter 30 by a precisely synchronized replica of the user code 34 which was originally used to spread the subscriber's data. This multiplication of the incoming samples by a user code is done serially, signal sample-by-signal sample, to produce despread samples which represent a despread signal.
To support diversity reception, or reception of multipath signals, each user may be allocated several demodulators 32 in order to recover all received signal energy from a particular subscriber. In the prior art, four demodulator circuits are allocated to each subscriber to form a 4-branch, or 4-finger, Rake receiver at the base station of the digital cellular system. In FIG. 1, multiple demodulator circuits are shown, each receiving a user code or a delayed copy of the user code, and each having an output coupled to a summer 36 which adds together the energy demodulated in each finger of the Rake receiver. The outputs of the summers 36 are coupled to decoders 38 which decode encoded symbols from summers 36 to produce user voice samples or user data. Decoder 38 may be implemented by a convolutional decoder, such as a Viterbi decoder.
Thus, one disadvantage in the prior art is the large number of duplicated circuitry required to provide the multi-finger Rake receiver for each subscriber. Another disadvantage in the prior art is that fingers of the Rake receiver are permanently allocated to a particular subscriber. Therefore, if one subscriber requires more than the fixed number of fingers and another subscriber does not need all of its allocated fingers, surplus fingers from one subscriber may not be allocated to another subscriber needing additional fingers.
Therefore, a need exists for an improved method and system for demodulating a code division multiple access signal which permits reallocation of demodulation resources and reduces the circuitry required to demodulate CDMA from one or more subscribers.